subphylum - vertebrata

Class – Chondrichthyes
They are marine animals with streamlined body and have cartilaginous endoskeleton (Figure).
Notochord is persistent throughout life.
Mouth is located ventrally.
Gill slits are separate and without operculum (gill cover).
The skin is tough, containing minute placoid scales.
Teeth are modified placoid scales which are backwardly directed.
Their jaws are very powerful. These animals are predaceous.
Due to the absence of air bladder, they have to swim constantly to avoid sinking.
Heart is two-chambered (one auricle and one ventricle).

Class – Osteichthyes
It includes both marine and fresh water fishes with bony endoskeleton. Their body is streamlined. Mouth is mostly terminal (Figure).
They have four pairs of gills which are covered by an operculum on each side. Skin is covered with cycloid/ctenoid scales.
Air bladder is present which regulates buoyancy. Heart is two chambered (one auricle and one ventricle).




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